1. Statement of the Technical Field
The present invention relates to the field of antennas, and more particularly to switchable sub-reflector antenna system using fluidic dielectrics.
2. Description of the Related Art
Typical satellite antenna systems use either parabolic reflectors or shaped reflectors to provide a specific beam coverage, or use a flat reflector system with an array of reflective printed patches or dipoles on the flat surface. These “reflect array” reflectors used in antennas are designed such that the reflective patches or dipoles shape the beam much like a shaped reflector or parabolic reflector would, but are much easier to manufacture and package on a spacecraft.
However, satellites typically are designed to provide a fixed satellite beam coverage for a given signal and may be limited in bandwidth by the structure of the reflectors and sub-reflectors. For example, Continental United States (CONUS) beams are designed to provide communications services to the entire continental United States. Once the satellite transmission system is designed and launched, changing the beam patterns to improve the operational bandwidth would be difficult.
The need to change the beam pattern provided by the satellite has become more desirable with the advent of direct broadcast satellites that provide communications services to specific areas and possibly on different frequency ranges. Without the ability to change beam patterns and coverage areas as well as to flexibly use multiple frequency ranges, additional satellites must be launched to provide the services to possible future subscribers, which increases the cost of delivering the services to existing customers.
Some existing systems are designed with minimal flexibility in the delivery of communications services. For example, a symmetrical Cassegrain antenna that uses a movable feed horn, defocuses the feed and zooms circular beams over a limited beam aspect ratio of 1:2.5. This scheme has high sidelobe gain and low beam-efficiency due to blockage by the feed horn and the subreflector of the Cassegrain system. Further, this type of system splits or bifurcates the main beam for beam aspect ratios greater than 2.5, resulting in low beam efficiency values. Other systems attempt to alter beam width and gain by using multiple feed horns. In any event, most of these systems will have a main reflected signal that will be interfered with by a sidelobe of the radiator or feed horn.
In another system as shown in FIG. 1, a dynamic reflector surface comprising an array of tunable reflective surfaces is used instead of a fixed reflector surface. Each element of the array can be tuned separately to change the phase during the process of reflection, and thus the beam pattern generated by the array of tunable reflectors can be changed in-flight in a simple manner. Each reflecting element in the array is a horn reflecting device which reflects an electric field emanating from a single feed horn. Each horn in the array has the capability of changing the phase during the process of incidence and reflection. This phase shift can then be used to change the shape of the beam emanating from the array. The phase shift can be incorporated by either using a movable short or by using a variable phase-shifter inside the horn and a short. By using “phase-shifting” which can be controlled on-orbit, a relatively simple reconfigurable antenna can be designed. This approach is much simpler than an active array in terms of cost and complexity.
More specifically, FIG. 1 illustrates a front, side, and isometric view of the existing horn reflect array as described in U.S. Pat. No. 6,429,823. Reflect array 200 is illuminated with RF energy from feed horn 202. Reflect array 200 comprises a plurality of reflective elements 204 that are configured in a reflector array 206. Side view 208 shows that feed horn 202 is pointed at the open end 210 of reflective element 204. Side view 208 also shows that reflector array 206 can be a curved array. Further, front view 212 and isometric view 214 show that reflective elements 204 can be placed in a circular arrangement for reflector array 206. Each reflective element 204 reflects a portion of the incident RF energy, and by changing the respective phase for each reflective element 204, the respective phase of the portion of the reflected RF energy for each respective reflective element 204 can be changed. By changing the phase of each portion of the reflected RF energy, different beam patterns can be generated by the horn reflect array. Although the reflector array 206 provides lower non-recurring costs for a satellite and can generate a plurality of different shaped beam patterns without reconfiguring the physical hardware, e.g., without moving the location of the feed horn 202 and the reflective elements 204 in the reflector array 206, the design is still too complicated to provide a simple mechanism able to switch a sub-reflector in and out of a reflection path. Reflect array 200 does not include a sub-reflector and would further require complex programming of reflective elements even if such elements were contemplated on a sub-reflector.
In any event, a programmable array such as the reflector array 206 can be reconfigured on-orbit. Satellites using the reflector array 206 can be designed for use in clear sky conditions, and, when necessary, the beams emanating from the reflector array 206 can be shaped to provide higher gains over geographic regions having rain or other poor transmission conditions, thus providing higher margins during clear sky conditions.
It can be seen, then, that there is a need in the art for an antenna system that can be alternatively reconfigured in-flight without the need for complex systems. It can also be seen that there is a need in the art for a communications system that can be reconfigured in-flight that has high beam-efficiencies and high beam aspect ratios. There is also a need for an antenna that is able to simply switch a sub-reflector on and off for use with multiple feed horns and that can optionally have the advantages of the antenna of FIG. 1 and other advantages as will be further described below utilizing fluidic dielectrics in accordance with the present invention.
Two important characteristics of dielectric materials are permittivity (sometimes called the relative permittivity or ∈r) and permeability (sometimes referred to as relative permeability or μr). The relative permittivity and permeability determine the propagation velocity of a signal, which is approximately inversely proportional to √{square root over (με)}. The propagation velocity directly affects the electrical length of a transmission line and therefore the amount of delay introduced to signals that traverse the line.
Further, ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to √{square root over (L1/C1)} where L1 is the inductance per unit length and C1 is the capacitance per unit length. The values of L1 and C1 are generally determined by the permittivity and the permeability of the dielectric material(s) used to separate the transmission line structures as well as the physical geometry and spacing of the line structures.
For a given geometry, an increase in dielectric permittivity or permeability necessary for providing increased time delay will generally cause the characteristic impedance of the line to change. However, this is not a problem where only a fixed delay is needed, since the geometry of the transmission line can be readily designed and fabricated to achieve the proper characteristic impedance. Analogously, wave propagation delays and energy beam patterns through dielectric materials in reflector and/or sub-reflector based antenna systems are typically designed accordingly with a fixed dielectric permittivity or permeability. When various time delays are needed for specific energy shaping or beam forming requirements, however, such techniques have traditionally been viewed as impractical because of the obvious difficulties in dynamically varying the permittivity and/or permeability of a dielectric board substrate material. Accordingly, the only practical solution has been to design variable delay lines using conventional fixed length RF transmission lines with delay variability achieved using a series of electronically controlled switches. Such schemes would be impracticable and overly complicated for a reflector or sub-reflector based antenna.